The overall goal of this project is to understand the mechanisms that govern death of neurons both in normal developmental and as a calamitous response to disease and injury. Understanding the former satisfies our curiosity and provides insight into how our nervous system is formed. Unraveling the latter may give us tools to treat neurodegenerative disorders, stroke and trauma. Our focus will be on the roles of transcription- dependent events and the pathways that regulate them in promoting neuron survival and death. Specifically, we will focus on two major pathways that mediate neuron death in a variety of circumstances relevant to both development and disease. These are the apoptotic "cell cycle" and "JNK" pathways. In each case, we will seek to extend (in both distal and proximal directions). our current knowledge about the component molecules that define and regulate these pathways. For the cell cycle pathway we will test the hypotheses that: a) The cell cycle regulatory protein Sei-1 plays a major role in initiating the pathway by interacting with and activating cyclin-dependent kinase 4 (Cdk4);b) Optimal promotion of neuron death occurs by a "coincidence" mechanism in which transcription factors regulated by the cell cycle, JNK and additional pathways simultaneously converge on promoters of pro-apoptotic proteins such as Bim and Puma. In contrast, the choice of the death proteins that are transcriptionally regulated (e.g. Bim vs. Puma) depends on the apoptotic stimulus and is dictated by a combinatorial transcription factor mechanism;c) The apoptotic proteins DP5(Hrk) and EglN3(Sm- 20) are also subject to regulation by the cell cycle pathway via the coincidence mechanism and these converge with Bim to promote efficient neuron death;and d) Components of the apoptotic cell cycle pathway uncovered in our cell culture models will also be relevant to neuron death caused by injury and disease and specifically, in Alzheimer's Disease.(AD). For the JNK pathway we will assess the hypotheses that: a) In healthy neurons, the apoptotic JNK pathway is suppressed by the POSH-related molecule POSH2 and the E3 ligase Cbl;that these promote destabilization of the required POSH scaffold protein and that in response to apoptotic stimuli, POSH2 and Cbl are lost/inactivated, leading to POSH stabilization and pathway initiation;b) Activation of the apoptotic JNK pathway requires translocation of its kinase components to the perinuclear region in a complex containing the scaffolds POSH and the JNK-interacting protein (JIP). In healthy neurons, JIP is localized to neurites and POSH levels are very low. In response to apoptotic stimuli, POSH is stabilized, binds JIPs and the complex (along with active JNKs) is actively translocated to the perinuclear region where it promotes death;and c) Components of the apoptotic JNK pathway uncovered in our cell culture models will also be relevant to neuron death caused by injury and disease and specifically, in Alzheimer's Disease (AD). These studies are anticipated to significantly enhance our understanding of how neurons die during development and in disease and to uncover potential targets and treatments to alleviate the latter.